Re: question about max_readahead for ide devices in 2.4?

[Andrew Morton <akpm@osdl.org>]

John Salmon <jsalmon@thesalmons.org> wrote:
>
> 
> 
> Several "tuning" recommendations suggest that sequential accesses of
> large files, and hence the performance of busy web servers, can be improved
> by changing the maximum readahead value with, e.g.,
> 
> echo 511 > /proc/sys/vm/max-readahead
> 
> But it looks to me like get_max_readahead in filemap.c ignores the
> value set by /proc/sys in favor of max_readahead[major][minor] whenever
> max_readahead[major] is non-NULL.  And furthermore that 
> max_readahead[major] IS initialized to non-NULL for ide devices in
> init_gendisk.  (N.B. I'm looking at 2.4 sources).
> 
> Conclusion: echoing a value into /proc/sys/vm/max-readahead won't change the
> readahead behavior for already-probed IDE devices.
> 
> Is this correct, or am I missing something?

That's correct - it's all a bit weird.   You should use

	blockdev --setra 511 /dev/hda

for IDE devices.  Not sure about scsi.  You may as well set
/proc/sys/vm/max-readahead to the same thing.